1. Field of the Invention
The present invention relates to thermoplastics films containing inorganic fillers, to metal/plastics composites containing inorganic fillers and to processes for the production thereof.
For manufacturing a can or a closure for use as packaging material, especially for packaging of foodstuffs, sheets of tinplate, chromated steel such as ECCS (electrolytic chromium-coated steel) and aluminum are coated in sheet or strip form. The coating layer acts as protective layer to protect the metal against the attack of the filled material and the corrosion resulting from this, on the one hand, and, on the other hand, to prevent corrosion products of the metal affecting the filled material. Naturally, the coating layer itself, must not leach coating components which would negatively effect the filled material, either during the sterilization of the filled material, which is carried out subsequent to the filling, or during the subsequent storage of the packaged goods, especially foodstuffs.
Furthermore, the coating components must be resistant to stresses occurring during the further processing of the coated sheets to form cans or closures, such as during deforming, stamping, crimping, beading and the like.
2. Description of the Related Art
The film coating of metal sheets has proven an advantageous process for the coating of sheets, especially for manufacturing foodstuff packagings. Thus, for example, German Laid-Open Applications 3128641 describes a process for producing laminates for foodstuffs packagings, in which the metal sheet and a thermoplastic resin film together with an adhesive based on a carboxyl group-containing polyolefin disposed between these layers are heated to temperatures above the melting point of the adhesive and then cooled together with the application of pressure, producing the metal/plastics composite.
Furthermore laminates and foodstuffs packaging containers, especially bags, produced from these laminates are known from German Laid-Open Application 2912023, GB-A-2027391 and EP-B-31701.
Opaque films of organic thermoplastics are known from EP-A-4633, which films are oriented by biaxial stretching and contain 1 to 25% by weight, based on the weight of the polymeric plastic, of inorganic particles, such as, for example, titanium dioxide, calcium carbonate and silicon dioxide. The inorganic particles lead to the opaque appearance of the plastics films.
Finally, EP-A-199 228 relates to composite films, which contain two plastics films bonded with adhesive, at least one of the two films being provided with a metal layer on its inner surface and one of the two plastics films having an opaque appearance on account of microvoids in the film, which are formed by inorganic fillers, such as sulfates, carbonates, silicates or oxides. The composite film is recommended for packaging of oxidation-sensitive foodstuffs and semi-luxury items. The opaque plastics film, which consists essentially of plastic as the main component and filler, is biaxially stretched. The transparent plastics film is also preferably stretched.
These plastics films have poor barrier properties in the non biaxially-stretched state, ie. the plastics films are water vapor permeable, acid permeable and generally gas permeable. As a result of this, undesired corrosion phenomena occur in the sterilization of preserve cans, eg. visible metal sulfide formations on the can.